1. Field
The presently disclosed embodiments relate generally to wireless telecommunications, and more specifically to a method and apparatus for soft handoff between base stations using different signaling frame formats.
2. Background
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and Amplitude Modulation (AM) schemes such as amplitude companded single sideband (ACSSB) are known in the art. These techniques have been standardized to facilitate interoperation between equipment manufactured by different companies. Code division multiple access communications systems have been standardized in the United States in Telecommunications Industry Association TIA/EIA/IS-95-B, entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEMS”, incorporated by reference herein, and hereinafter referred to as IS-95-B. The IS-95-B standard incorporates the IS-95-A, J-STD-008, and TSB74 standards.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled “The cdma2000 ITU-R RTT Candidate Submission. The Telecommunications Industry Association is currently developing the cdma2000 proposal as interim standard TIA/EIA/IS-2000, and hereinafter referred to as cdma2000. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission”, also known as “Wideband CDMA” and hereinafter referred to as W-CDMA. A third proposal was submitted by U.S. TG 8/1 entitled “The UWC-136 Candidate Submission”, hereinafter referred to as EDGE. The contents of these submissions is public record and is well known in the art.
The Signaling Layer 2 Link Access Control (LAC) signaling protocol architecture and functionality used to provide the transport and delivery of Open Systems Interconnection (OSI) Layer 3 signaling messages over cdma2000 radio channels is described in TIA/EIA/IS-2000.4-A entitled “Signaling Link Access Control (LAC) Standard for cdma2000 Spread Spectrum Systems”, hereinafter referred to as the LAC Standard. The LAC layer is a sublayer of OSI Layer 2. The LAC layer provides correct delivery of signaling messages comprised of signaling frames. Functions provided by the LAC sublayer comprise assured delivery, unassured delivery, duplicate signaling message detection, address control to deliver a signaling message to an individual mobile station, segmentation of signaling messages into suitably sized fragments for transfer over a physical medium, reassembly and validation of received signaling messages, and Global Challenge Authentication.
On the transmit side, the LAC layer receives a LAC Service Data Unit (SDU) from Layer 3 Signaling Services to be transmitted over a logical channel. The SDU is operated on by up to 5 LAC component layers to create an encapsulated LAC Protocol Data Unit (PDU). The encapsulated LAC PDU is segmented into fragments, and the fragments are passed to the Medium Access Control (MAC) layer (also a sublayer of OSI Layer 2) for channel mapping and access to the Physical Layer (OSI Layer 1). On the receive side, the MAC layer supplies LAC PDU fragments to the LAC layer, which reassembles them into complete signaling messages and passes the signaling messages up through the LAC component layers to the Layer 3 Signaling Services. The five component layers of the LAC are the Authentication Sublayer, the Automatic Repeat Request (ARQ) Sublayer, the Addressing Sublayer, the Utility Sublayer, and the Segmentation and Reassembly (SAR) Sublayer.
The format of the PDU has changed with certain releases of the CDMA standards for reasons such as added functionality and increased message efficiency. Each time the PDU format is changed with the release of a new standard, an associated Protocol Revision number is incremented. Protocol Revision numbers are associated with PDU formats supported by mobile stations (MOB_P_REV numbers), and with PDU formats supported by base stations (P_REV numbers). Table 1. specifies the Protocol Revision numbers currently planned or in use, associated by standard. The PDU format for each PDU revision number is specified in the LAC Standard.
TABLE 1StandardP_REVMOB_P_REVJ-STD 008P_REV 1MOB_P_REV 1IS-95-AP_REV 2MOB_P_REV 2IS-95-A + TSB 74P_REV 3MOB_P_REV 3IS-95-B Phase 1P_REV 5MOB_P_REV 4IS-95-B Phase 2P_REV 5MOB_P_REV 5IS-2000 Release 0P_REV 5MOB_P_REV 5IS-2000 Release AP_REV 6MOB_P_REV 7
Soft handoff in a CDMA communications system requires combining the contents of two or more receive paths from multiple base stations or multiple sectors of the same base station. The contents of differently formatted PDUs having unlike P_REV numbers cannot be combined during soft handoff. When the receive paths cannot be combined, a successful soft handoff between base stations supporting different PDU P_REVs cannot occur. Such a soft handoff failure could occur during a handoff wherein the origination base station is using a previous, or lower, P_than an alternative base station accepting the handoff, or wherein the origination base station is using a subsequent, or higher, P_REV than the alternative base station. Thus, there is a need in the art for soft handoff between base stations using different signaling frame formats, or P_REV numbers.